1. Field of the Invention
The present invention relates generally to the field of mapping electrical activity of a brain. More particularly, the present invention relates to a topographic map and a method for creating the map in which brain activities occurring in respective temporal lobe regions of a brain are displayed adjacent to each other and brain activities occurring in respective mid-line regions of the brain are displayed remotely.
2. Discussion of the Related Art
Creating topographic maps of brain electrical activity using a set of electrodes disposed on the scalp of a subject is well-known in the art. The electrical topographic map is created by measuring an electric field sensed by each electrode, converting these electric field measurements into processable signals and then locating the processed signals on a two dimensional map. Since the development of brain mapping, the resultant topographic map has always been displayed in a format that corresponds to a top-down or superior view, i.e., looking down on the top of the head.
FIG. 1 shows this basic top-down view 10. A set of electrodes 12i, were i varies from l to n is typically placed on the subject's scalp in accordance with the International 10-20 Electrode Placement System. FIG. 1 illustrates the placement of the elementary set of 10-20 electrodes. In the 10-20 electrode system, the patient's scalp is measured and then proportionally divided up in order to place electrodes 12i equally spaced over the scalp. Since the electrodes are equally spaced, the signals sensed by the electrodes can, after processing, be located on the grid of a map. Although occasionally, a flat side view or a three dimensional side view may also be used, the top-down view has been universally accepted as the standard throughout the field of neurophysiology.
The top-down view has several disadvantages. First, if the area of interest for mapping of brain activity is in the temporal lobes, this activity will be located on the outer peripheral edges 14 and 16 of the topographic map. The information so placed becomes less useful diagnostically because it appears to the physician looking at the map that some information may be missing or has been cut off. Furthermore, from a diagnostic perspective, since the majority of focal brain seizures occur in the temporal lobes (approximately 75% of all seizures in epileptic patients, for example) this important diagnostic information is difficult to interpret.
Additionally, information from sphenoidal electrodes has not been adequately incorporated into conventional topographic maps such as the map of FIG. 1. A sphenoidal electrode is a wire which is inserted by a physician into the jaw muscle and nearby tissue to a position near the sphenoid bone where it remains during a test period to sense epileptiform discharges. More specifically, the site of insertion is approximately 3-4 mm below the zygoma and 2-3 cm in front of the tragus and the target is the foramen ovale.
The sphenoidal electrodes are very sensitive to temporal lobe epileptic discharges and often reflect the maximum epileptic electroencephalogram activity. When this important diagnostic information has been used in conventional topographic maps of brain activity, this information has not been anatomically correctly integrated into the top-down topographic map. Anatomically, the sphenoidal electrodes sense brain electrical activity in an area that is substantially directly below the mid-temporal electrodes T3 and T4. Thus, the diagnostic information concerning the mesial part of the temporal lobe as sensed by the sphenoidal electrodes is either not depicted at all or depicted in a way that makes it diagnostically less useful.
Therefore, an object of the present invention is to provide an improved topographic map and method of mapping brain electrical activity that provides enhanced diagnostic information.
Another object of the present invention is to provide an improved topographic map and method of mapping brain electrical activity that permits mapping of the entire surface of the brain in one continuous plane.